1) Field of the Invention
The present invention relates to toners containing resin particles having an inner portion of resin material with a relatively high molecular weight and an outer portion of resin material having a relatively lower molecular weight, and to methods for producing the toner resins by using emulsion polymerization followed by suspension polymerization techniques. More specifically, this invention relates to the use of an emulsion copolymer as a seed for the formation of particles in suspension polymerization.
2) Description of the Related Art
Styrene-acrylate and styrene-methacrylate copolymers have been used as toner resins for many years, and there are different methods known for preparing such resins.
Suspension polymerization is considered one of the easiest methods for preparing toner resins. Styrene-acrylate, styrene-methacrylate and styrenebutadiene copolymers prepared by suspension polymerization are some of the most common examples of toner resins. Emulsion copolymerization is another method for preparing toner resins (e.g. U.S. Pat. No. 5,683,847), but this method is applied less frequently than suspension polymerization is applied.
Many toner formulations utilize mechanical mixtures of polymers with different polarities and molecular weight. It is known that high molecular weight polymers (HMWP) have good mechanical properties but they are difficult to process. Low molecular weight polymers (LMWP) typically have excellent processability, but the products from them possess diminished mechanical properties. The mechanical blend of a HMWP with a LMWP was an obvious option to attempt to combine the major advantages of both materials. Unfortunately, such a blend is usually characterized by flow instability in the molten state. This is due to the difficulty in mixing together, on a molecular level, polymers with considerably different molecular weights. To improve the interpenetration of polymer chains, a longer processing time in a molten state would be required. However, extended exposure to high temperatures and oxygen leads to undesirable degradation. Thus, methods whereby a polymer blend with good processability and excellent mechanical properties may be obtained are still being sought. It has been taught that the individual high and low molecular weight components, forming a bimodal distribution of molecular weight (DMW), should meet certain requirements to produce adequate electrostatic images. For example, it is claimed in U.S. Pat. No. 5,716,746 that if the low molecular weight (LMW) component has a molecular weight less than 5,000, the anti-blocking properties of the toner may be lowered and fogging during the development process may occur. On the other hand, if the LMW component has a molecular weight above 20,000, then the low temperature fixing performance may become poor. Similar restrictions apply also to the high molecular weight (HMW) component. If the HMW component has a molecular weight below 200,000, anti-blocking and anti-offset properties are lowered. If the HMW component has a molecular weight above 1,000,000, toner viscosity increases thereby making the low-temperature fixing performance poor. Magnetic toner resins with a bimodal distribution are, thus, produced according to U.S. Pat. No. 5,716,746 by first synthesizing a LMW component and subsequently synthesizing a HMW component within these limits.
Toner resins with bimodal and trimodal distribution of molecular weight are prepared according to U.S. Pat. No. 5,738,964 by blending of individual components in xylene solution and evaporation of solvent. Bimodal resins also have been obtained by two subsequent solution polymerizations (U.S. Pat. No. 5,750,301).
U.S. Pat. Nos. 5,736,288 and 5,744,276 describe the preparation of a toner resin with a bimodal DMW. The low molecular component is prepared by a solution copolymerization in xylene. The high molecular portion is prepared by a separate suspension polymerization or in bulk. Both components are combined in xylene and isolated after evaporation of solvent. A modification of above procedure is given in the U.S. Pat. No. 5,310,812, in which the low molecular component is first prepared in solution, isolated from the solvent and dissolved in monomers used for suspension polymerization of the high molecular component.
A toner resin with bimodal DMW is prepared according to U.S. Pat. No. 4,968,574 by two subsequent suspension polymerizations. In the first suspension polymerization, benzoyl peroxide (BPO) initiator is used in the amount of 0.2% on monomers producing a HMW component. A LMW component is then produced by swelling the beads with a subsequent portion of monomers with 4% of BPO. A similar two-step suspension polymerization process is described in U.S. Pat. Nos. 5,071,918 and 5,204,413. The HMW component is prepared in those instances by using a multifunctional initiator and the LMW component is prepared by use of a higher amount of BPO.
More recently, in U.S. Pat. No. 5,852,147 a styrene-acrylic copolymer with a bimodal DMW was prepared by a combination of two subsequent emulsion copolymerizations. In the first step, a HMW latex is prepared using a very small amount of chain transfer agent. The prepared emulsion is transferred into a second reactor after the addition of a fresh portion of monomers, an initiator and a high amount of mercaptan chain transfer agent (5% on monomers). The second step emulsion copolymerization is then carried out. The resulting latex has a bimodal DMW, as shown by a GPC measurement.
Schlund et al. (Polymer, 1989, Vol. 60, October, pp. 1883-94) reported on films made from polymer particles having core-shell, multilayer of homogeneous structures. Thermomechanical analysis of the corresponding films suggests that two or more phases exist in the core-shell or multilayered latex particles. Hergeth et al. (Polymer, 1989, Vol. 60, October, pp. 1913-17) reported conditions for forming shells on polymeric seeds for a two-stage emulsion polymerization technique.
More recently, U.S. Pat. No. 5,928,830 described latex processes for preparation of a polymer resin wherein emulsion polymerization techniques are used to prepare a low molecular weight core. A shell of substantially higher molecular weight second polymer is then formed by emulsion polymerization over the core particles.
New, useful and economical methods for making toner resins having a suitable combination of HWM and LMW components to form resins with bimodal distribution of molecular weight having good processability and excellent mechanical properties are still being sought.